A high voltage bushing is conventionally used for passing an electrical conductor through a pressure vessel wall of, for example, a large generator, without allowing hydrogen gas inside the pressure vessel to leak out of the vessel. The conductor must be electrically insulated from the pressure vessel wall, and this is achieved by enclosing the conductor inside a porcelain sleeve or tube. An annular, sleeve-like metallic mounting flange is telescoped over the exterior surface of the porcelain sleeve and is utilized to attach the porcelain sleeve to the pressure vessel wall. One such high voltage bushing is illustrated in FIGS. 1-3. Bushings of this type, however, have been known to experience cracks during operation, the cause of which is presently not known with certainty. High voltage bushings used on large generators are rated for high currents and the conductor is directly cooled with gas or liquid. These high currents cause heat generation in the flange resulting in the flange having a higher temperature than the porcelain. The metallic flange also has a higher coefficient of thermal expansion than the porcelain insulator. It is thought that the combination of these two factors produce thermal stresses in the porcelain sleeve which are high enough to cause the porcelain to crack. Another possibility is that the high forces seen during a sudden short circuit can also generate stresses in the porcelain sleeve sufficient to cause cracks.